1. Field of the Invention
The present invention relates to an air-fuel ratio control apparatus and an air-fuel ratio control method for an internal combustion engine. More particularly, the invention relates to air-fuel ratio control apparatus and method for an internal combustion engine for controlling an influent exhaust gas average air-fuel ratio to a target value.
2. Description of the Related Art
The ratio of the total amount of air to the total amount of reducing agents and fuel supplied into an intake passage, a combustion chambers and a portion of an exhaust passage extending upstream of a given location in the exhaust passage is termed the air-fuel ratio of exhaust gas passing by the location. As a related technology, internal combustion engines are known which are designed to burn a lean air-fuel mixture and which have in exhaust passages thereof NOx absorbents that absorb NOx when the air-fuel ratio of influent exhaust gas is on a leaner than a theoretical air-fuel ratio and that release absorbed NOx when the oxygen concentration in influent exhaust gas decreases to or below a certain level. In these internal combustion engines, the air-fuel ratio of exhaust gas flowing into the NOx absorbent is temporarily shifted to the richer side of the theoretical air-fuel ratio to release NOx from the NOx absorbent. The released NOx is then reduced.
However, since the fuel and lubricants used in internal combustion engines contain sulfuric substances, exhaust gas from these engines contains sulfuric substances, for example, SOx or the like. SOx is absorbed into the NOx absorbent, in the form of, for example, SO.sub.4.sup.2-, together with NOx. However, SOx absorbed in the NOx absorbent cannot be released therefrom merely by shifting the air-fuel ratio of exhaust gas flowing into the NOx absorbent to the fuel-richer side. Therefore, the amount of SOx in the NOx absorbent gradually increases and, as the amount of SOx absorbed in the NOx absorbent increases, the NOx absorbing capability of the absorbent decreases and, eventually, the NOx absorbent becomes substantially unable to absorb NOx. However, SOx absorbed in the NOx absorbent may be released in the form of, for example, SO.sub.2, by decreasing the oxygen concentration in exhaust gas flowing into the NOx absorbent when the temperature of the NOx absorbent is relatively high. Thus, a known emission control apparatus causes a NOx absorbent to release SOx by temporarily shifting the air-fuel ratio of exhaust gas flowing into the NOx absorbent to the theoretical air-fuel ratio or to the richer side thereof while heating the NOx absorbent.
If exhaust gas flowing into the NOx absorbent contains a large amount of oxygen and a large amount HC at the same time, the oxygen and the HC react on the NOx absorbent, so that reaction heat is produced and the NOx absorbent is heated. A related-art emission control apparatus utilizing this phenomenon is described in, for example, Japanese Patent Application Laid-Open No. HEI 8-61052. In this apparatus, a plurality of engine cylinders are divided into a first cylinder group and a second cylinder group. The emission control apparatus causes SOx absorbed in a NOx absorbent to be released therefrom by setting the air-fuel ratio of the mixture to be burned in the first cylinder group to the richer side to produce exhaust gas containing a large amount of HC, and setting the air-fuel ratio of the mixture to be burned in the second cylinder group to the leaner side to produce exhaust gas containing a large amount of oxygen. The exhuast gas from both the first and second cylinder groups is then simultaneously introduced into the NOx absorbent to heat the NOx absorbent, and the average air-fuel ratio of the influent exhaust gas is set to the theoretical air-fuel ratio or to the richer side thereof so that SOx is released from the NOx absorbent.
In order to efficiently utilize oxygen and HC flowing into the NOx absorbent to heat the NOx absorbent, it is necessary to keep the influent exhaust gas average air-fuel ratio at the theoretical air-fuel ratio or slightly to the richer side thereof. Therefore, in the aforementioned emission control apparatus, an air-fuel ratio sensor for detecting the influent exhaust gas average air-fuel ratio is provided in a portion of the exhaust passage upstream of the NOx absorbent. Based on an output signal of the air-fuel ratio sensor, the apparatus controls the amounts of fuel injected into the first and second groups of cylinders so that the influent exhaust gas average air-fuel ratio becomes equal to a target value, for example, the theoretical air-fuel ratio.
In the aforementioned emission control apparatus, however, since the air-fuel ratio sensor is disposed upstream of the NOx absorbent in the exhaust passage, a large amount of HC comes into contact with the air-fuel ratio sensor, and therefore produces a large amount of hydrogen (H.sub.2). Therefore, there is a danger that the air-fuel ratio sensor will covered with a large amount of H.sub.2. If the air-fuel ratio sensor is covered with H.sub.2, the contact of the air-fuel ratio sensor with oxygen carried in the exhaust gas becomes less likely, so that the air-fuel ratio sensor may falsely detect that the influent exhaust gas average air-fuel ratio is on the richer side. Based on this false detection, the amounts of fuel to be injected into the first and second groups of cylinders will be controlled so that the influent exhaust gas average air-fuel ratio is shifted to the leaner side although this operation is actually not needed. Thus, the related-art emission control apparatus has a problem of false control of the influent exhaust gas average air-fuel ratio.